Silicon carbide (SiC) is a semiconductor material with high hardness, of which the band gap is larger than the band gap of silicon (Si), and is applied to various semiconductor devices, such as a power element, an environmentally-resistant element, a high-temperature operation element, and high-frequency element. Among these elements, application to power elements, such as a semiconductor element and a rectifier element is gaining attention. Power elements using SiC has an advantage that they can significantly reduce power loss, compared to power elements using Si. Further, SiC power elements allow fabrication of smaller semiconductor devices, since having such an advantage as described above, than Si power elements.
One of typical semiconductor elements among power elements using SiC is a metal-insulator-semiconductor field-effect transistor (MISFET). Hereinafter, the MISFET using SiC may be simply referred to as a “SiC-FET.” A metal-oxide-semiconductor field-effect transistor (MOSFET) is one type of MISFETs.
Using the SiC-FET as a switching element for an electric power converter that drives and controls a load (e.g., a motor), for example, has been considered. In the case of using a MISFET as a switching element of an electric power converter, a freewheeling current may flow when the MISFET is in the off state. In commonly-used inverter circuits, a freewheeling diode is externally connected in antiparallel with the MISFET to ensure a path of the freewheeling current. In the case where the SiC-FET is applied to an inverter circuit, a Schottky diode made of SiC is selected as a freewheeling diode.
On the other hand, the MISFET includes a pn-junction in its structure, and functions as a diode between source and drain, and therefore called a “body diode.” If a current can flow using the pn-junction existing in the SiC-FET when the channel of the MISFET is in the off state, it is possible to omit a freewheeling diode as an external device in the case where the MISFET is applied to an inverter circuit. As a result, the number of parts can be reduced. It is reported, however, that if a forward current flows in the SiC pn-junction, a problem unique to SiC occurs, that is, stacking faults increase due to basal plane dislocation. Using the pn-junction diode (i.e., a body diode) existing in the SiC-FET as a freewheeling diode causes a current to flow in the body diode, which is the pn-junction, in a forward direction. If such a current flows in the SiC pn-junction, it is expected that crystal deterioration of the SiC-FET (e.g., an increase in the stacking faults in the pn-junction) may proceed due to bipolar operation of the body diode (see, e.g., Patent Document 1).
On-state voltage of the body diode may increase when the crystal deterioration of the SiC-FET proceeds. Further, in the case where the body diode is used as a freewheeling diode, a reverse recovery current flows when the diode shifts from the on state to the off state, due to the bipolar operation of the pn-junction diode. The reverse recovery current causes a recovery loss, and leads to a reduction in switching speed.
As mentioned above, in the SiC-FET, a current flowing in the body diode may cause element deterioration due to an increase in recovery loss and an increase in stacking faults. It is therefore impossible to use the body diode as a freewheeling diode.
The inventors of the present application therefore previously invented a SiC-FET in which the channel structure of the SiC-FET is optimized and includes a function of diode in a channel portion, thereby allowing a current to flow from a source electrode to a drain through the channel portion that includes the diode function, without allowing the current to flow through the body diode, when the SiC-FET is in the off state (see Patent Document 2). With this structure, the external freewheeling diode can be omitted when the SiC-FET is applied to an inverter circuit. It is therefore possible to reduce the number of parts of the inverter circuit.